1. Field of the Invention
This invention relates to an optical fiber amplifier and a dispersion compensating fiber module for use with an optical fiber amplifier.
2. Description of the Related Art
In recent years, research and development of an optical communication system has been and is being performed energetically, and the importance of booster amplifiers, repeaters or preamplifiers which make use of the technique of optical amplification in which an erbium (Er) doped fiber (an erbium-doped-fiber may be hereinafter referred to as “EDF”) is employed has become apparent.
Further, due to the appearance of optical amplifiers, attention is drawn to an optical-amplifier-repeated transmission system since the transmission system plays a very important role in achievement of economization of a communication system in the multimedia society.
By the way, in an ordinary rare earth doped fiber optical amplifier which particularly amplifies a wavelength of a signal, the length of the doped fiber is set to a value at which a maximum gain is obtained in order to assure a high conversion efficiency from pump power to signal power.
Meanwhile, in a wavelength division multiplexing (WDM) optical amplifier which amplifies many channels at the same time, it is important to keep the wavelength dependency of the gain as flat as possible. As a result, the rare earth doped fiber (which will be hereinafter discussed in connection with a representative EDF) must operate in a condition wherein the degree of the saturation of the gain is low. To this end, where the concentration of high level ions is represented by N2 while the concentration of all ions is represented by N1 and N2/N1 is defined as pump ratio, in order to raise the average pump ratio N2/N1 of the doped fiber over the entire length, the length of the doped fiber must be set short.
However, if the doped fiber is formed short in this manner, then much residual pump power will leak out from the other end of the doped fiber, resulting in degradation of the conversion efficiency. Nevertheless, since the required pump power increases as the number of signal wavelengths increases, the output power of a semiconductor pump laser must be raised.
In particular, although it is apparent from the conservative law of energy that the pump power increases as the number of wavelengths increases, a wavelength multiplexing optical amplifier cannot be used in a condition in which it exhibits a high efficiency of conversion from pump power to signal power. This is because, since the rare earth doped fiber is intentionally formed short so as to prevent saturation in order to obtain a gain over a wide bandwidth or to make the gain flat, pump power which has not been converted into a signal will leak out from the other end of the doped fiber.
Accordingly, while high pump power is required originally when comparing with ordinary amplification of only one signal channel, the rare earth doped fiber must be used in a condition wherein the pump power leaks out therefrom.
Thus, in order to effectively make use of thus leaking out residual pump light, a technique has been proposed wherein a reflecting mirror is provided at the other end of a doped fiber so that residual pump light is reflected by the reflecting mirror so as to be introduced back into the doped fiber so that it may be used for optical amplification again. The technique is disclosed in Japanese Patent Laid-Open Application No. Heisei 3-25985 or Japanese Patent Laid-Open Application No. 3-166782.
However, where residual pump light is reflected by the reflecting mirror in this manner, the pump light is returned not only to the doped fiber but also to the pump source. This pump light may possibly give rise to unstable operation of the pump source such as interference.
By the way, while, due to the appearance of optical amplifiers, attention is drawn to an optical-amplifier-repeated transmission system which includes a plurality of repeating and amplifying optical amplifiers since it plays a very important role in achievement of economization of a communication system in the multimedia society as described above, the transmission system has subjects to be solved in terms of the dispersion compensation, reduction in nonlinear effects (effects having a bad influence on the transmission quality) in an optical fiber serving as a transmission line and economic wide bandwidth wavelength multiplexing transmission.
Generally, an optical fiber serving as a transmission line has a dispersion characteristic and accumulates a dispersion amount in proportion to the length thereof. Usually, however, in an optical fiber transmission system which employs regenerative repeaters, the dispersion amount is reset at the regenerative repeaters. Consequently, the accumulation of the dispersion amount does not make a problem.
However, in an optical-amplifier-repeated transmission system, since a transmitted optical signal is repeated by a kind of analog amplification, the dispersion amount is accumulated. Accordingly, in order to eliminate the accumulation, the signal wavelength used for transmission should be set to a zero dispersion wavelength. This, however, provides the following subjects to be solved:
1-1) Optical fibers have already been laid by a large amount, and unfortunately, those optical fibers have a zero dispersion wavelength at 1.3 μm while an optical amplifier which is expected to be put into practical use soon can amplify only a signal of the 1.55 μm band;
1-2) It has been reported recently that, even if optical fibers whose zero dispersion wavelength is 1.55 μm are laid newly to transmit a signal of 1.55 μm, nonlinear effects occur actively in the optical fibers. This signifies that, if a signal wavelength equal to a zero dispersion wavelength is used for transmission, then undesirable nonlinear effects occur; and
1-3) Particularly in wavelength multiplexing transmission, since a plurality of different signal wavelengths are involved, the concept that the signal wavelengths are set equal to a zero dispersion wavelength cannot be applied.
Accordingly, it has been proposed recently to intentionally displace the signal wavelength from the zero dispersion wavelength suitably and compensate for the dispersion, for example, at the repeater.
While research of dispersion compensators has been and is being performed actively in recent years in this manner, one of dispersion compensators which is expected to be most likely put into practical use is a dispersion compensating fiber (which may be referred to as “DCF”; here the term DCF is the abbreviation of Dispersion Compensating Fiber). The DCF, however, has the following subjects to be solved:
2-1) Where fibers (transmission lines) laid already are utilized, a dispersion compensating fiber must be interposed as a device at each repeating point in order to perform dispersion compensation collectively at such each repeating point. Therefore, research and development is being directed to reduction in length of dispersion compensating fibers.
2-2) When fibers are to be laid newly, it is a possible idea not to interpose a dispersion compensating fiber as a device but to lay a dispersion compensating fiber as part of a transmission line. For example, a transmission line of 40 km may be formed from a fiber of 20 km and a dispersion compensating fiber of 20 km. However, research and development of such a novel dispersion compensating fiber as just mentioned makes overlapping development with research and development of a dispersion compensating fiber for the application described in paragraph 2-1) above.
In summary, in wavelength multiplexing transmission, a wavelength dispersion must be compensated for, and since the compensation for a wavelength dispersion is expected to be most likely put into practical use where a dispersion compensating fiber is employed, it is prospective to use a dispersion compensating fiber. Further, it is investigated to incorporate a dispersion compensating fiber as a part into an optical amplifier repeater. Generally, however, the mode field diameter of a dispersion compensating fiber (DCF) is set small in order to compensate for a dispersion, and consequently, nonlinear effects are liable to occur and, as the dispersion amount to be compensated for increases, also the loss increases.
Thus, it is a possible method to compensate also for the loss of a dispersion compensating fiber using an optical amplifier. In this instance, the loss must be compensated for so that a transmission optical signal may not be influenced by nonlinear effects which degrade the quality of a signal such as self-phase modulation (SPM) and cross-phase modulation (XPM) occurring in the dispersion compensating fiber. Accordingly, the possible method has a problem in that designing of a level diagram is difficult. Further, while a flat and wide optical amplification bandwidth is required for an optical amplifier for WDM, also a rare earth doped fiber optical amplifier has a wavelength dependency of the gain. Accordingly, there is a subject to be solved in that it is difficult to realize a flat and wide amplification bandwidth.
Meanwhile, a rare earth doped fiber optical amplifier having a high gain sometimes suffers from unnecessary oscillations which are produced when it performs optical amplification. If such unnecessary oscillations are produced, the rare earth doped fiber optical amplifier operates but unstably.
For example, in an erbium-doped-fiber optical amplifier, spontaneous emission light (ASE) of 1.53 to 1.57 μm in wavelength is generated when optical amplification is performed, and since the ASE is repetitively reflected from reflection points in the erbium-doped-fiber optical amplifier, unnecessary oscillations are liable to be produced. Particularly with an erbium-doped-fiber optical amplifier adjusted for multiple wavelength collection amplification (that is, an erbium-doped-fiber optical amplifier having a high pump rate), since it has a high gain in the proximity of 1.53 μm, unnecessary oscillations are liable to be produced at this wavelength. When such unnecessary oscillations are produced, the erbium-doped-fiber optical amplifier operates but unstably.